In the ongoing process of cell differentiation, cell phenotypes appear to change at times of discrete developmental transitions. Certain of these transitions are preprogrammed and autonomous while others are dependent upon hormonal, or other stimuli. We propose to analyze the developmental transitions as they occur within a model system. The model is the insect exocrine glands which produce the seminal plasma and spermatophore of the mealworm beetle. The paired accessory glands of male mealworm beetles originate as mesodermal rudiments in the last instar larva, undergo organogenesis before pupal ecdysis, pass through phases of cell proliferation and cell specialization, and following adult ecdysis, exhibit an increase in biosynthetic capacity and a phase of peak differentiation. Our previous work has provided a number of biochemical and morphological indices by which the extent of differentiation can be scored. The differentiation occurs within a well-defined endocrine context of fluctuating titres of beta-ecdysone, bursicon, and juvenile hormone. The first experimental series will use ligatures to assess the importance of anterior neuroendocrine centers in the achievement of developmental transitions. In vivo culture will allow more precise definition of the minimum requirements for developmental transitions, and replacement therapy (addition of hormones and neuroendocrine centers to cultures) will be used to establish specific dependence. Finally, we will attempt to pinpoint the time of determination for autonomous transitions and to define the minimum patterns of macromolecular synthesis, either as pre-requisites or co-requisites, which are required for each transition. In addition to their significance as general models of cytodifferentiation these glands which produce the seminal plasma, offer good models of reproductive maturation in long-lived holometabolous insects, and thus provide background for the design and evaluation of strategies for the control of insect vectors of disease.